Your practice nurse walks by with a cooler full of vaccines that she is returning to public health. “The refrigerator’s temperature went out of range last night ... again.” You adjust the temperature on the refrigerator but the thermostat just does not allow for fine enough adjustments.
Stability data demonstrate that freezing (< 0°C) most refrigerated vaccines destroys them.1 Additionally, while all vaccines cumulatively lose potency with time, the loss in potency is accelerated in a vaccine-specific manner when they are stored above the recommended temperature.1 Thus, frozen vaccine is usually discarded, and vaccines that have been exposed to temperatures above recommended ranges should be marked and, depending on the particular preparation and the extent of exposure, used before other stock.
For these reasons suppliers use purpose-built vaccine refrigerators to keep stock between 2°C and 8°C. Some pharmaceutical-grade refrigerators2 have features such as digital temperature control, forced air circulation, and alarms, with a rated temperature stability of ± 1°C. Such refrigerators, however, are expensive.
Home beer makers also have strict temperature requirements for their products. After watching a YouTube video on how to wire a digital temperature controller added to a domestic refrigerator for brewing use,3 it seemed reasonable to add a digital temperature controller to a refrigerator used for vaccine storage.
Materials
The following materials are needed:
4-ft3 cooler-only refrigerator (about $200);
3/16-inch flat screwdriver;
14-gauge power cable;
STC-1000 digital temperature controller (purchased online for about $20);
plastic or metal enclosure (about $8);
drill and file;
wire strippers;
soldering iron and tin; and
digital maximum-minimum thermometer (about $20) to measure vaccine temperature with the probe submerged in a vial of glycol, as indicated by the Centers for Disease Control and Prevention.4
Considerations
Put your refrigerator on bypass.
Using the temperature controller as a bypass requires setting up the controller to intermittently power an electrical plug for a domestic refrigerator when the air probe temperature is higher than the set point. The refrigerator in turn is set to run at its coldest setting and is plugged into the controlled plug.
We took an enclosure (in this case a recycled computer power supply), gutted its original contents, and cut a rectangular hole to receive the controller. Then wiring was attached to the controller from the incoming power cord to a switched plug (Figures 1 and 2).
The wiring of a bypass temperature controller
Schematic and pinout of a bypass temperature controller
The calibration of the temperature probes was tested against ice water as a reference.
The digital thermometer probe was run past the gasket at the top of the refrigerator door and placed in the centre of the refrigerator measuring the air temperature. The finished assembled bypass is shown in Figure 3.
Assembled bypass controller
Optional improvements.
We put a standard 12-volt computer case fan (about $20) inside the refrigerator to reduce temperature variation. We found that the variation between the bottom and top of the refrigerator decreased from 5°C to 2.5°C with fan operation at a cost of wider temperature swings with door-opening events.
We use water bottles in nonusable high- and low-temperature areas of the refrigerator (crisper, door, and top and bottom shelves) to add thermal ballast. The water bottles displace 10% of the rated refrigerator volume and slow internal heating to 0.04°C/min when the refrigerator turns off owing to power failures; this extends the time that the refrigerator keeps vaccines under 8°C during power failures to more than an hour (about $4 for 10 L of water bottles).
Clinic’s experience with the technique and settings.
We keep our temperature controller set to 3.8°C (air) with cooling cycling after a 1°C change. Over a month’s time, this kept the vaccine thermometer between 3.5°C and 7.2°C. (With the refrigerator’s original mechanical thermostat, the previous month’s vial temperature ranged between 2.4°C and 7.7°C.)
We had not experienced any cold-chain failures from the time of installation of the digital temperature controller to the time of manuscript submission to Canadian Family Physician.
Conclusion
Adding a digital thermometer controller to your refrigerator is a practical approach to controlling temperature variations in domestic refrigerators, potentially to the level of pharmaceutical-grade tolerances. Those who are uncomfortable with their wiring skills can consider prewired digital bypass controllers that are available (about $170).
Acknowledgments
I thank Dr David Page for his input throughout the study and for reviewing the manuscript draft.
Notes
We encourage readers to share some of their practice experience: the neat little tricks that solve difficult clinical situations. Praxis articles can be submitted online at http://mc.manuscriptcentral.com/cfp or through the CFP website (www.cfp.ca) under “Authors and Reviewers.”
Footnotes
Competing interests
None declared
- Copyright© the College of Family Physicians of Canada
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